The present invention relates to an apparatus and method for venting pressure from closed systems. More particularly, the invention relates to an apparatus and method for venting vacuum pressure from a semiconductor wafer etcher in order to prevent damage to a turbo pump in the etcher.
In the fabrication of semiconductor devices, particularly sub-micron scale semiconductor devices, profiles obtained in the etching process are very important. Careful control of a surface etch process is therefore necessary to ensure directional etching. In conducting an etching process, when an etch rate is considerably higher in one direction than in the other directions, the process is called anisotropic. A reactive ion etching (RIE) process assisted by plasma is frequently used in an anisotropic etching of various material layers on top of semiconductor substrate. In plasma enhanced etching processes, the etch rate of a semiconductor material is frequently larger than the sum of the individual etch rates for ion sputtering and individual etching due to a synergy in which chemical etching is enhanced by ion bombardment.
To avoid subjecting a semiconductor wafer to high-energy ion bombardment, the wafer may also be placed downstream from the plasma and outside the discharge area. Downstream plasma etches more in an isotropic manner since there are no ions to induce directional etching. The downstream reactors are frequently used for removing resist or other layers of material where patterning is not critical. In a downstream reactor, radio frequency may be used to generate long-lived radioactive species for transporting to a wafer surface located remote from the plasma. Temperature control problems and radiation damage are therefore significantly reduced in a downstream reactor. Furthermore, the wafer holder can be heated to a precise temperature to increase the chemical reaction rate, independent of the plasma.
In a downstream reactor, an electrostatic wafer holding device known as an electrostatic chuck is frequently used. The electrostatic chuck attracts and holds a wafer positioned on top electrostatically. The electrostatic chuck method for holding a wafer is highly desirable in the vacuum handling and processing of wafers. An electrostatic chuck device can hold and move wafers with a force equivalent to several tens of Torr pressure, in contrast to a conventional method of holding wafers by a mechanical clamping method.
Referring initially to FIG. 1, a conventional inductively coupled plasma etched chamber 10 is shown. In the etch chamber 10, which typically represents one that is commercially available as a LAM TCP etcher, the plasma source is a transformer-coupled source that generates a high density, low pressure plasma away from a wafer surface. The plasma source allows an independent control of ion flux and ion energy. The plasma can be generated by a flat spiral coil (not shown), i.e., an inductive coil separated from the plasma by a dielectric plate 12 which is normally fabricated of a ceramic material with a gas inlet 14 provided therein. The dielectric plate 12 may be a dielectric window made of a substantially transparent material such as quartz to facilitate visual observation of a middle chamber 20. The middle chamber 20 is further formed by a bottom ceramic plate 16 equipped with an opening 18 for allowing a plasma to pass thereto. The sidewall 22 of the middle chamber 20 is normally formed of a metallic material, such as aluminum, with an anodized aluminum surface. The top ceramic plate 12, the bottom ceramic plate 16 and the metallic sidewall 22 form a self-contained chamber, i.e., the middle chamber 20 which has a first cavity 24 therein.
A wafer 30 is positioned on an electrostatic chuck (or ESC) 26, inside the chamber interior 33 of the main chamber 32, sufficiently away from the RF coil (not shown) such that it is not affected by the electromagnetic field generated by the RF coil. A typical LAM TCP plasma etcher enables a high density plasma to be produced and a high etch rate to be achieved. In a typical inductively coupled RF plasma etcher 10, an inductive supply and a bias supply are further used to generate the necessary plasma field. In a typical inductively coupled RF plasma etcher 10, shown in FIG. 1, a source frequency of 13.5 MHZ and a substrate bias frequency of 13.5 MHZ are utilized such that ion density of about 0.5-2.0xc3x97102 cm3 is obtained at the wafer level, while electron temperature of about 3.5-6.0 eV and a chamber pressure of 1-25 mTorr are achieved.
In the plasma chamber 10, after the wafer 30 is etched in a main chamber 32, etchant gas is normally evacuated from the middle chamber 20 and from the main chamber 32 by a turbo pump 34 controlled by a gate valve 36. The turbo pump 34 is further connected to a dry pump 38 through an isolation valve 42 and connecting conduits 40 and 44. Opening of the isolation valve 42 allows the etchant gas to be evacuated from the turbo pump 34 by the dry pump 38. Simultaneous with the etchant gas pumping process, an inert purge gas such as nitrogen is introduced into the middle chamber 20 and the main chamber 32 through the gas inlet 14 to further facilitate the removal of residual etchant gas from the chamber interiors 24 and 33, respectively.
In the conventional plasma chamber 10, over time etchant gas flowing through the main chamber 32, the middle chamber 20 and the gate valve 36 frequently causes corrosion of the internal components of these elements. Accordingly, the internal components of the main chamber 32, the middle chamber 20 and the gate valve 36 must periodically be cleaned and repaired for routine maintainence of the plasma etch chamber 10. This is accomplished by initially closing the gate valve 36 in order to vent residual etchant gas from the plasma etch chamber 10 typically by operation of a chamber auto-vent 28, manually opened and closed by a lever 29. This action causes atmospheric or ambient air to enter the chamber interior 24 of the middle chamber 20 and the chamber interior 33 of the main chamber 32, and the air pressure in the chamber interiors 24, 33 equalizes with the atmospheric or ambient air. Simultaneously, the turbo pump 34 generates an internal vacuum pressure which is isolated by the closed gate valve 36 and the isolation valve 42. After maintenance of the main chamber 32, the middle chamber 20 and/or the gate valve 36, the gate valve 36 is manually re-opened to equalize pressure between the middle chamber 20 and the turbo pump 34. Due to the relatively large difference in pressure between the chamber interiors 24, 33 and the internal vacuum generated by the turbo pump 34, typically about 760 Torr, air contained in the chamber interiors 24, 33 rushes through the now-open gate valve 36 and into the turbo pump 34, which is prone to damage due to the rushing air. Therefore, an apparatus is needed for equalizing pressure between the chamber interior 24 of the middle chamber 20 and the turbo pump 34 prior to opening the gate valve 36 in order to prevent damage to the turbo pump 34.
Accordingly, an object of the present invention is to provide an apparatus and method for preventing vacuum-induced damage to a turbo pump in a plasma etch chamber or system for semiconductors.
Another object of the present invention is to provide an apparatus and method for venting pressure from a closed system.
Another object of the present invention is to provide an apparatus and method for dispelling or reducing pressure differentials in a system.
Still another object of the present invention is to provide an apparatus and method for reducing a pressure differential between a chamber and a pump separated from the chamber by a valve before opening the valve.
Yet another object of the present invention is to provide an apparatus and method for gradually dispelling vacuum pressure from a closed system in order to prevent damage to internal components of the system.
Still another object of the present invention is to provide an apparatus and method for raising pressure in a turbo pump from a vacuum pressure to atmospheric or near-atmospheric pressures before opening a gate valve between the turbo pump and a chamber in order to equalize or nearly equalize pressure between the air or gas pressure in the turbo pump and the air or gas pressure in the chamber and prevent damage to the turbo pump.
A still further object of the present invention is to provide a venting apparatus which can be retrofitted to a variety of closed vacuum systems including but not limited to plasma etch systems having a turbo pump, a gate valve and an isolation valve.
Yet another object of the present invention is to provide a plasma etch chamber which includes a venting apparatus for selectively venting vacuum pressure from the chamber.
In accordance with these and other objects and advantages, the present invention includes a method and apparatus comprising a purge conduit and vent conduit attached to a turbo pump of a plasma etch chamber. The purge conduit may communicate with atmospheric air or with a nitrogen source or clean, dry air (CDA) source, and the vent conduit is fitted with a manual valve, an electric valve, or both, along with a flow restrictor and an end cap provided with an air or gas vent. The air flow restrictor facilitates gradual, rather than rapid, escape of air or gas from the chamber, through the turbo pump and from the vent conduit upon opening a gate valve between the chamber and the turbo, to prevent damage to the internal turbo pump components.